U.S. patent number 5,073,403 [Application Number 07/460,353] was granted by the patent office on 1991-12-17 for aluminum-plated steel sheet for cans.
This patent grant is currently assigned to NKK Corporation. Invention is credited to Hiroshi Kagechika, Hiroshi Kibe, Tadahiko Mishima.
United States Patent |
5,073,403 |
Kagechika , et al. |
December 17, 1991 |
Aluminum-plated steel sheet for cans
Abstract
The invention relates to a plated steel sheet for cans which
must have high workability and corrosion resistance and can prevent
bimetallic corrosion and a method of manufacturing the same. The
plated steel sheet is manufactured by forming an electroplated
chromium layer on the surface of a steel, removing a hydrated
chromium oxide layer formed on the surface of the chromium layer,
and forming an aluminum plating layer, so that the electroplated
chromium layer and the aluminum plating layer are stacked in direct
contact with each other. Another plated steel sheet is manufactured
by forming a vacuum deposited chromium layer on the surface of a
steel, and forming an aluminum plating layer.
Inventors: |
Kagechika; Hiroshi (Tokyo,
JP), Mishima; Tadahiko (Tokyo, JP), Kibe;
Hiroshi (Tokyo, JP) |
Assignee: |
NKK Corporation (Tokyo,
JP)
|
Family
ID: |
27339285 |
Appl.
No.: |
07/460,353 |
Filed: |
January 3, 1990 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
280147 |
Dec 2, 1988 |
4906533 |
Mar 6, 1990 |
|
|
Current U.S.
Class: |
427/528; 427/404;
427/250; 427/405; 427/531 |
Current CPC
Class: |
C25D
11/38 (20130101); C25D 5/48 (20130101) |
Current International
Class: |
C25D
5/10 (20060101); C25D 5/48 (20060101); C25D
11/00 (20060101); C25D 5/12 (20060101); C25D
11/38 (20060101); C23C 014/32 (); C23C 014/30 ();
C23C 014/16 () |
Field of
Search: |
;427/38,42,50,250,404,405 ;204/192.31,192.12,192.15
;428/651,653 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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45-5123 |
|
Feb 1970 |
|
JP |
|
45-19762 |
|
Jul 1970 |
|
JP |
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46-4047 |
|
Feb 1971 |
|
JP |
|
46-25608 |
|
Jul 1971 |
|
JP |
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46-39445 |
|
Nov 1971 |
|
JP |
|
46-42006 |
|
Dec 1971 |
|
JP |
|
51-097544 |
|
Aug 1976 |
|
JP |
|
59-32544 |
|
Aug 1984 |
|
JP |
|
21139 |
|
Jan 1988 |
|
JP |
|
Other References
Bunshah et al., Deposition Technologies for Films and Coatings,
(Noyes Publications, Park Ridge, N.J.), c. 1982, pp. 267-268. .
Shunzo Miyazaki, "Internal Corrosion of Food Cans" in Iron and
Steel;, vol. 73, No. 3, 1987..
|
Primary Examiner: Beck; Shrive
Assistant Examiner: Burke; Margaret
Attorney, Agent or Firm: Frishauf, Holtz, Goodman &
Woodward
Parent Case Text
This is a division of application Ser. No. 07/280,147 filed Dec. 2,
1988, now U.S. Pat. No. 4,906,533, issued Mar. 6, 1990.
Claims
What is claimed is:
1. A method of manufacturing an aluminum-plated steel for cans,
comprising the steps of:
preparing a steel sheet;
forming a chromium layer on the surface of said steel sheet by ion
plating; and
forming an aluminum layer on said chromium layer by plating.
2. A method according to claim 1, wherein
said step of forming said aluminum layer is selected from the group
consisting of vapour deposition, ion plating, and a combination
thereof.
3. A method according to claim 1, comprising preheating said steel
sheet to about 200.degree. C. prior to ion plating of said chromium
layer on the surface thereof.
4. A method according to claim 3, wherein said steel is preheated
to about 200.degree. C. in a vacuum.
5. A method according to claim 3, comprising cooling said steel
sheet with said chromium layer thereon, prior to forming of said
aluminum layer.
6. A method according to claim 1, comprising preheating said steel
sheet prior to forming said chromium layer thereon.
7. A method of according to claim 6, further comprising cooling
said steel sheet with said chromium layer thereon, prior to forming
of said aluminum layer on said chromium layer.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a plated steel sheet used for cans
such as food cans and, more particularly, to a plated steel sheet
suited to food cans adopting an aluminum easy-open top.
2. Description of the Prior Art
Tin-plated steel sheet, tin free steel (obtained by forming a
chromium plating layer on the surface of a steel sheet and forming
a hydrated chromium oxide layer thereon), and aluminum plates have
been conventionally widely used as can materials. As easy-open tops
are increasingly used for drink cans, full-open end cans adopting
an aluminum easy-open top have been recently used for food cans.
Easy-open cans of this type can be conveniently opened without a
can opener and therefore are strongly demanded. For this reason, a
demand has arisen for supply of inexpensive and reliable can
materials.
Conventionally, both a can top and a can body of a full-open end
can are made of aluminum. Aluminum is, however, more expensive than
a tin-plated steel sheet or a chromium-plated steel sheet, and its
strength is unsatisfactory. Therefore, aluminum is damaged during
handling, or defective cans are sometimes produced. In addition,
although aluminum has a good corrosion resistance to general food,
its corrosion resistance to highly corrosive can contents
containing a large amount of salt such as salted food or food
cooked with soy sauce is not satisfactorily reliable.
In consideration of the above situation, a method has been proposed
in which properties of both aluminum and steel are utilized, i.e.,
soft aluminum is used as an easy-open top and a surface-treated
steel sheet having strength and an under film corrosion resistance
is used as a can body which must have strength so that a corrosion
resistance against a can content is obtained by a paint coated on
the inner surface of the can. One of a can body and a can top made
of different materials is selectively dissolved and corroded, i.e.,
a problem of so-called bimetallic corrosion is posed. The
bimetallic corrosion is a phenomenon in which when two types of
metals having different electrode potentials are placed in the
presence of an electrolyte and are electrically brought into
contact with each other, both the metals serve as electrodes to
form a cell, a current flows between the metals from a relatively
noble one to a base one through a contact point therebetween, and
the base metal is ionized and dissolved. When a can top is made of
aluminum and a can body is made of a tin-plated steel sheet,
aluminum serves as a base metal and tin serves as a noble metal.
Therefore, aluminum is ionized by an anode reaction, and hydrogen
is produced on the surface of tin plating by a cathode reaction. If
the aluminum top has a film defect, this defect portion is locally
dissolved, and a hole is produced by pitting. At the same time, a
film on the tin plating is peeled by hydrogen produced at the
cathode to corrode the tin-plated steel sheet. This phenomenon
similarly occurs in tin free steel. Especially when chlorine ions
are contained in a can content, the aluminum top turns to a base
metal more easily, and the phenomenon occurs more
significantly.
In order to prevent such bimetallic corrosion, a method of
increasing the strength of a film coated on the inner surface of a
can is studied, but a cost is inevitably increased in this method.
In addition, a method is studied in which a potential behavior of
an aluminum top is examined to make some improvements in an
aluminum alloy designing step (see, for example, "Iron and Steel",
1987, Vol. 3, PP. 427 to 436). This method is, however, not
practically used yet.
Aluminum can be plated on a steel sheet by conventional techniques.
Examples of the conventional techniques are a method of
manufacturing an aluminum single layer-plated steel sheet utilizing
vapour deposition (Japanese Patent Publication Nos. 45-5123,
45-19762, 46-39445 and 59-32544) and a method of manufacturing a
steel sheet having different metals, i.e., aluminum as an upper
layer and Ti, Cr or Zn as a lower layer formed thereon (Japanese
Patent Publication Nos. 46-4047, 46-25608 and 46-42006). Both of
these methods, however, aim at improving a corrosion resistance of
a steel sheet such as resistance to sprayed salt water but do not
aim at using such a plate as a can material. Therefore, in these
methods, an under film corrosion resistance is not taken into
consideration at all.
As described above, an aluminum-plated steel sheet aiming at
improving a general corrosion resistance to serve as a can body
material of a convenient full-open can have a problem of an under
film corrosion resistance. On the other hand, a tin-plated steel
sheet or tin free steel as a conventional can material having an
under film corrosion resistance poses a problem of bimetallic
corrosion.
SUMMARY OF THE INVENTION
It is, therefore, a first object of the present invention to
provide a plated steel sheet for cans in which no bimetallic
corrosion occurs between the steel sheet and an aluminum top and
which has a high under film corrosion resistance.
It is a second object of the present invention to provide a plated
steel sheet for cans which can be manufactured at low cost.
In order to achieve the above objects of the present invention,
there is provided an aluminum-plated steel sheet for cans
manufactured by forming an electroplated chromium layer having a
thickness of 0.005 .mu.m to 0.05 .mu.m without a hydrated chromium
oxide layer on the surface of a steel sheet and forming an aluminum
plating layer having a thickness of 0.01 .mu.m or more thereon. In
addition, according to the present invention, there is provided a
method of manufacturing a plated steel sheet, comprising the steps
of: forming a chromium plating layer having a thickness of 0.005 to
0.05 .mu.m on the surface of a steel sheet by electroplating and at
the same time forming a hydrated chromium oxide layer on the
surface; removing the hydrated chromium oxide; and coating aluminum
on the surface of the electroplated chromium layer, from which the
hydrated chromium oxide layer is removed, to a thickness of 0.01
.mu.m or more.
According to the plated steel sheet for cans of the present
invention, the brittle hydrated chromium oxide layer is removed,
and then the aluminum plating layer is directly formed on the
electroplated chromium layer. Therefore, the steel sheet which
maintains its high under film corrosion resistance even after it is
formed into cans and in which no bimetallic corrosion occurs
between the steel sheet and an aluminum top can be provided at low
cost.
Another plated steel sheet for cans according to the present
invention is manufactured by sequentially forming a vacuum
deposited chromium layer having a thickness of 0.1 to 0.7 .mu.m, an
aluminum layer having a thickness of 0.05 to 0.4 .mu.m, and an
aluminum chemical conversion layer on a steel sheet. A thickness
ratio of the aluminum layer to all the layers is 0.2 to 0.7.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic sectional view showing a plated steel sheet
for cans according to the present invention; and
FIG. 2 is a schematic sectional view showing another plated steel
sheet for cans according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A plated steel sheet of the present invention shown in FIG. 1
comprises electroplated chromium layer 2 having a thickness of
0.005 .mu.m to 0.05 .mu.m and formed on the surface of steel sheet
1, and aluminum plating layer 3 having a thickness of 0.01 .mu.m or
more and formed on the surface of layer 2. The aluminum plating
layer is a layer for eliminating a potential difference in a can
and preventing bimetallic corrosion of an aluminum top and must be
formed to a thickness of 0.01 .mu.m or more so as to uniformly
cover the entire steel sheet surface. A preferable upper limit of
the thickness of the aluminum plating layer is 5 .mu.m. A
composition of the aluminum layer is the same as that of pure
aluminum or an aluminum material of an easy-open top. If aluminum
is directly plated on a steel sheet, an electrode potential
difference between aluminum and steel is increased. Therefore, even
a small detect of a plated film forms a cell between the plating
layer and the steel sheet in a can. As a result, bimetallic
corrosion easily occurs to degrade an under film corrosion
resistance of the plating layer. In order to solve the above
problem, according to the steel sheet of the present invention,
chromium plating layer 2 is formed between steel sheet 1 and
aluminum plating layer 3. Since an electrode potential of chromium
is intermediate between aluminum and steel, the potential
difference between aluminum and chromium is reduced. Therefore, the
bimetallic corrosion between the plating layer and the steel can be
prevented to maintain the high under film corrosion resistance. In
addition, the chromium plating layer can galvanically protect steel
against corrosion. Therefore, even if the plating layer has a
defect, local corrosion at this place can be suppressed. Even a
thin chromium plating layer has a good corrosion resistance. In
addition, since a mass production technique is established for
electroplating of chromium, inexpensive products can be promisingly
supplied. If the thickness of the electroplated chromium layer is
less than 0.005 .mu.m, a satisfactory under film corrosion
resistance cannot be obtained. A thickness exceeding 0.05 .mu.m is,
however, economically disadvantageous. When chromium is plated by
electroplating, a hydrated chromium oxide layer is simultaneously
formed on the chromium plating layer. This hydrated chromium oxide
layer is brittle and therefore is often destroyed during a plated
steel sheet manufacturing process. Therefore, if aluminum is plated
on the hydrated chromium oxide layer, a satisfactory adhesive
property of the film cannot be obtained. For this reason, the
electroplated chromium layer should not have the hydrated chromium
oxide layer. In a method of the present invention, the hydrated
chromium oxide layer formed by electroplating is removed before
aluminum is plated. This removing treatment is performed by a
dipping treatment using an alkaline solution, plasma sputtering, or
a combination of both. In a dissolving method using an alkaline
solution as an example of the removing treatment, a steel sheet
having a hydrated chromium oxide formed thereon is dipped in a 40
g/l caustic alkali solution at 80.degree. C. for 30 seconds, rinsed
with water, and dried. In an electrolytic removing method, a steel
sheet is dipped in a 50 g/l chromate solution at 50.degree. C. so
that the steel sheet is electrolyzed to be 5 A/dm.sup.2 for 15
seconds, and then rinsed with water and dried. In plasma
sputtering, a steel sheet is exposed to an RF plasma of 5 kW in an
Ar +H.sub.2 (20%) atmosphere at 2.times.10.sup.-2 Torr for ten
minutes. With these removing treatments, the hydrated chromium
oxide layer can be efficiently removed without adversely affecting
the chromium plating layer.
According to the above method, an electroplated chromium layer from
which a brittle hydrated oxide layer is removed is formed on the
surface of a steel sheet, and an aluminum layer is directly stacked
on this layer. Therefore, a plated steel sheet for cans which has
high workability and under film corrosion resistance and does not
cause bimetallic corrosion can be easily obtained.
Another plated steel sheet according to the present invention shown
in FIG. 2 comprises chromium layer 12, and aluminum layer 13
sequentially formed on the surface of steel sheet 11. The plated
surface of this steel sheet is used as the inner surface of a can.
The chromium layer can be formed by vapour deposition, sputtering,
ion plating or the like. Of these methods, ion plating is
advantageous in uniformity and an adhesive property. The film
thickness of the chromium layer is 0.1 to 0.7 .mu.m, and
preferably, 0.2 to 0.5 .mu.m. A film thickness range is thus
limited because if the film is too thin, under film corrosion
occurs; if it is too thick, workability and an adhesive property
are degraded. A composition of the chromium layer is not limited to
pure chromium but may be an alloy containing various components in
an amount not degrading the characteristics of chromium. Similar to
the chromium layer, the aluminum layer can be plated by various
physical methods. In consideration of a plating rate, however,
vapour deposition or ion plating is preferred. The film thickness
of the aluminum layer is 0.05 to 0.4 .mu.m, and preferably, 0.1 to
0.3 .mu.m. This is because if the film is too thin, bimetallic
corrosion may occur; if it is too thick, under film corrosion may
occur. A composition of the aluminum layer is not limited to pure
aluminum but may be an alloy containing various components in an
amount not degrading characteristics of aluminum. Preferably, the
composition of aluminum is identical to that of aluminum used as a
can top material. The aluminum chemical conversion layer is, if
necessary, formed on the aluminum layer to further increase an
under film corrosion resistance. A treating method for this layer
comprises such a conventional aluminum chemical conversion
treatment that a phosphate treatment, a chromate treatment or a
phosphoric acid/chromic acid treatment is performed by dipping in a
treating solution, spraying of a treating solution or electrolysis
in a treating solution. The thickness of the chemical conversion
layer is normally about 0.01 to 0.1 .mu.m.
The chromium layer of the present invention effectively,
significantly suppresses expansion of local corrosion at a cracked
or pore portion. In addition, the aluminum layer causes the
potential of a can body made of the steel sheet according to the
present invention to be equal to that of an aluminum top, thereby
preventing bimetallic corrosion. In such a steel sheet having the
chromium layer and the aluminum layer, if the aluminum layer is too
thick, a large amount of blisters may be produced after painting to
promote under film corrosion. In the present invention, however,
since the thickness of the aluminum layer is limited to the above
range so that the layer becomes relatively thin, production of
blisters can be prevented. Moreover, since the aluminum and
chromium layers are stacked, aluminum and chromium are partially
alloyed when a can is manufactured by welding. As a result, a
melting point is lowered to improve weldability as compared with
that obtained when only a chromium layer is formed on a steel
sheet. Furthermore, since a thin aluminum layer is formed in the
present invention, weldability is better than that obtained when a
thick aluminum layer is formed on a steel sheet.
The present invention will be described in more detail below by way
of its examples. In the following description, Examples 1 to 3
correspond to the steel sheet shown in FIG. 1; and Example 4, the
steel sheet shown in FIG. 2.
EXAMPLE 1
A commercially available tin-plated steel sheet was prepared. This
steel had a chromium plating layer formed on its surface and a
hydrated chromium oxide layer formed on the surface of the chromium
plating layer. The steel was dipped in a 2 N potassium hydroxide
solution at 85.degree. C. for five minutes. Then, the steel was
subjected to DC plasma sputtering using Ar plasma of 5 kV at
10.sup.-2 Torr for ten minutes, thereby removing a hydrated
chromium oxide layer formed on the steel surface. In this
treatment, the chromium plating layer was not adversely affected.
Then, aluminum was vacuum-deposited on the steel surface from which
the hydrated chromium oxide layer was removed using an electron
beam for heating a deposition source at a vacuum degree of
10.sup.-3 Torr, a steel temperature of 250.degree. C., and a
deposition rate of 0.01 .mu.m/sec, thereby manufacturing steel
plates (Nos. 1 to 4) each having an aluminum layer formed on the
chromium plating layer. The thicknesses of both the layers are
shown in Table 1.
An under film corrosion resistance of each plated steel sheet
manufactured as described above was estimated by an accelerated
test, and a corrosion state in a can and bimetallic corrosion
thereof were estimated by a real can test. The under film corrosion
resistance was estimated as follows. That is, 50 mg/dm.sup.2 of an
epoxyphenol paint was coated on the plated steel sheet and baked at
205.degree. C. for ten minutes. Thereafter, a cross cut was made to
reach the underlying steel surface by a knife, and the resultant
material was subjected to 5-mm stretch forming by an Erichsen
testing machine, thereby preparing a test piece. The test piece was
dipped in a corrosive liquid containing 1.5 wt % of salt and 1.5 wt
% of citric acid and having a pH of 3.0 at 70.degree. C. for 20
hours. Thereafter, an adhesive tape was adhered on the film surface
and then peeled, and a corrosion width and a film peeled state at
this time were observed. The real can test was performed as
follows. That is, the plated steel sheet was formed into a can
body, a bottom plate was added thereto, and a boiled salmon piece
was put into the can. Then, the can was vacuum-packed using an
aluminum easy-open top to prepare a full-open end canned food. The
canned food was preserved at 37.degree. C. for two months.
Thereafter, a corrosion state in the can was observed to estimate a
sulfur blackening resistance and a bimetallic corrosion resistance.
The result is shown in Table 1.
EXAMPLE 2
A steel sheet was dipped and electroplated at a current density of
50 A/dm.sup.2 for 0.2 to 0.8 minutes in a chromic acid bath having
a composition of 150 g/l of anhydrous chromic acid and a liquid
temperature of 40.degree. C. As a result, an electroplated chromium
layer was formed on the surface of the steel sheet, and a hydrated
chromium oxide layer was formed on the surface of this layer. The
hydrated chromium oxide layer which was naturally formed was
removed by plasma sputtering following the same procedures as in
Example 1. Then, Al was vacuum-deposited on the surface of the
electroplated chromium layer following the same procedures as in
Example 1, thereby preparing plated steel sheets (Nos. 5 to 7). The
prepared plated steel sheets were tested following the same
procedures as in Example 1. The result is shown in Table 1.
EXAMPLE 3
A steel sheet was dipped and electroplated at a current density of
50 A/dm.sup.2 for 0.2 minutes in a sulfuric acid bath having a
composition of 150 g/l of anhydrous chromic acid and a liquid
temperature of 40.degree. C. As a result, an electroplated chromium
layer was formed on the surface of the steel sheet, and a hydrated
chromium oxide layer was formed on the surface of this layer. The
hydrated chromium oxide layer which was naturally formed was
removed by plasma sputtering following the same procedures as in
Example 1. Then, Al was vacuum-deposited on the surface of the
electroplated chromium layer following the same procedures as in
Example 1, thereby preparing a plated steel sheet (No. 8). The
prepared plated steel sheet was tested following the same
procedures as in Example 1. The result is shown in Table 1.
For purposes of comparison of Examples 1 to 3, plated steel sheets
(Nos. 9 and 10) as comparative examples in which an electroplated
chromium layer without a hydrated chromium oxide layer, and an
aluminum layer were formed on the surface of a steel sheet but the
aluminum layer was thinner than that of the present invention, and
plated steel sheets (Nos. 11 and 12) in which only an aluminum
layer was formed on the surface of a steel sheet and tin free steel
(No. 13) as conventional examples were tested following the same
procedures as in Example 1. The test result is shown in Table
1.
As shown in Table 1, the plates of Comparative Examples Nos. 9 and
10 had poor bimetallic corrosion-n resistances because the upper
aluminum plating layer was thinner than 0.01 .mu.m. Of the
conventional examples, the aluminum single layer-plated steel
sheets (Nos. 11 and 12) had poor results in a cross cut test and a
bimetallic corrosion resistance. This means that the under film
corrosion resistance was unsatisfactory and the aluminum plating
layer covering the surface before the test was degraded in the real
can test. The tin free steel (No. 13) was found to have a good
under film corrosion resistance because the result of the cross cut
test was good but had a poor bimetallic corrosion resistance. In
contrast, the examples (Nos. 1 to 8) of the present invention
achieved good or very good results in all the tests.
TABLE 1 ______________________________________ Section Type and
Thickness of Plating Layer Real Can Test Inner Outer Cross Sulfer
Bimettalic layer Layer Cut Blackening Corrosion No. Cr Al Total
Test Resistance Resistance ______________________________________
Example 1 1 0.01 0.02 0.03 .largecircle. .largecircle.
.largecircle. 2 0.01 5.3 5.31 .largecircle. .circleincircle.
.circleincircle. 3 0.03 0.03 0.06 .circleincircle. .largecircle.
.largecircle. 4 0.03 6.3 6.33 .largecircle. .circleincircle.
.circleincircle. Example 2 5 0.01 0.15 0.16 .largecircle.
.largecircle. .circleincircle. 6 0.01 8.6 8.61 .largecircle.
.circleincircle. .circleincircle. 7 0.03 0.72 0.75 .circleincircle.
.circleincircle. .circleincircle. 8 0.01 5.8 5.83 .circleincircle.
.circleincircle. .circleincircle. Comparative Example 9 0.01 0.005
0.015 .largecircle. .DELTA. X 10 0.03 0.002 0.032 .circleincircle.
.largecircle. X Conventional Example 11 -- 0.5 0.5 X .DELTA. X 12
-- 8.0 8.0 X .largecircle. .DELTA. CrOX .circleincircle.
.largecircle. X 13 0.015 0.01 0.025
______________________________________ *: Example 3
.circleincircle.; excellent, .largecircle. ; good, .DELTA.; poor,
X; unsatisfactory
EXAMPLE 4
A solvent-degreased 0.32-mm thick cold rolled steel plate was
preheated to 200.degree. C. in a vacuum of 6.times.10.sup.-6 Torr.
Then, chromium was deposited on the steel sheet, and aluminum was
deposited thereon. After the resultant steel sheet was cooled to
room temperature, it was dipped in a commercially available
aluminum chemical conversion solution (phosphoric acid-chromic acid
solution) and then rinsed with water and dried. The thicknesses of
the respective layers of the prepared plated steel sheet are shown
in Table 2.
50 mg/dm.sup.2 of an epoxyphenol paint was coated and baked (at
205.degree. C. for ten minutes) on the plated steel sheet and a
cross cut was formed therein. Then, the plated steel sheet was
subjected to 5-mm stretch forming using an Erichsen testing
machine. Thereafter, the resultant steel sheet was dipped in a
solution mixture (pH =3.0) of 1.5% of NaCl and 1.5% of citric acid
(at 70.degree. C. for 20 hours) and then rinsed with water and
dried. Then, a tape peeling test was performed to estimate under
film corrosion. The result was shown in Table 2.
A full-open end can was manufactured using the above steel sheet as
a can body and aluminum as a can top, and a solution similar to
that in the tape peeling test was filled therein as an imitation
solution, thereby performing a real can test for three months. The
result is shown in Table 2.
TABLE 2 ______________________________________ No. Al Cr Al/Cr + Al
UFC BMC ______________________________________ Comparative Example
1 1.0 -- 1.0 x .smallcircle. Comparative Example 2 0.6 0.5 0.55 x
.smallcircle. Present Invention 3 0.3 0.3 0.50 .smallcircle.
.smallcircle. Present Invention 4 0.3 0.6 0.33 .smallcircle.
.smallcircle. Present Invention 5 0.1 0.4 0.20 .smallcircle.
.smallcircle. Present Invention 6 0.1 0.2 0.33 .smallcircle.
.smallcircle. Comparative Example 7 0.03 0.2 0.13 x x Comparative
Example 8 -- 0.5 0 x x ______________________________________ UFC:
under film corrosion BMC: bimetallic corrosion o: corrosion is
absent, under film property is good x: corrosion is present, under
film property is poor
* * * * *